your query must be:
SELECT v_name, v_minval, v_maxval, v_defval
FROM table1
WHERE $X{IN, v_name, MyParam}
GROUP BY v_name, v_minval, v_maxval, v_defval
ORDER BY v_name,v_minval,v_maxval,v_defval
The only way that sql can return the whole table is if you select all the possible values in the input control or all the values for column v_name are the same and you choose that value.